1. The total number of specific dexamethasone-binding sites in rat heart and liver cytosol was measured at intervals after adrenalectomy.
2. Between 12 and 48 h after adrenalectomy there was a significant increase in the number of binding sites in both heart and liver cytosol. Affinity was unchanged.
3. Of the [3H]corticosterone bound to liver cytosol proteins after an intravenous injection 98% disappeared within 2 h in vivo. Dissociation of endogenous corticosterone-receptor complexes in liver cytosol will thus be substantially complete some hours before the number of receptors increases.
4. It was concluded that there is a true increase in the number of glucocorticoid receptors occurring principally between 12 and 48 h after suppression of endogenous steroids.